1. Field of the Invention:
This invention relates generally to a compact fluorescent lamp provided with a screw base, and more particularly, to the disposition of a ballast relative to a winding envelope.
2. Description of the Prior Art:
Fluorescent lamps have been used as a general source of illumination for many years. A fluorescent lamp has high lighting efficiency and a low consumption of electric power in comparison with an incandescent lamp and therefore a compact fluorescent lamp provided on an incandescent lamp base, i.e. an E 26 - type screw base, has been developed. But in order to interchange an incandescent lamp with such a compact fluorescent lamp, it is necessary for the fluorescent lamp to have a folded envelope because, in order to obtain about the same brightness as with an incandescent lamp, it is necessary for the fluorescent lamp's envelope to have a certain minimum length. Such a compact fluorescent lamp is known, for example from U.S. Pat. Nos. 3,953,761 and 4,199,708. Such a fluorescent lamp has a lamp base providing a plurality of screw thread portions, a screw base which is secured to the thread portions of the lamp base, a globe, a partition plate dividing the base side and the globe side, a winding envelope and a ballast provided to the partition plate, in which screw threads are formed on both the lamp base and the screw base itself. However, as the screw base is mounted directly to the lamp base in a conventional lamp, it is necessary for the lamp base to be subjected not only to a torsion moment when attaching to a socket but also to the total weight of the lamp including a heavy ballast, a winding envelope and other equipment attached by way of the partition plate. So it is desirable that the connection between the screw base and the lamp base be modified.
Moreover, in such a fluorescent lamp the ballast and the winding envelope are closely disposed and are covered with the globe so that the temperature of the ballast occasionally reaches around 100.degree. C. The winding envelope is designed to exhibit a maximum luminous efficiency when the coolest wall temperature of the winding envelope is about at 40.degree. C. However, the coolest wall temperature exceeds 40.degree. C. and reaches over 60.degree. C. on account of the radiant heat of the ballast. The luminous efficiency is therefore reduced by about 25% on account of the raising of the coolest wall temperature of the winding envelope. It has been considered to dispose the ballast away from the winding envelope, so as to avoid overheating the winding envelope, but this is undesirable as the fluorescent lamp as a whole then becomes large-sized.
Moreover, according to the lighting direction (i.e. downward lighting with the base-up, upward lighting with the base-down, or side lighting with the base to the side), the coolest wall temperature of the winding envelope will be different. Especially in the case of upward lighting, the coolest wall temperature is most apt to be influenced by the heat of the ballast, and the luminous efficiency shows a declining tendency. Thus, such a fluorescent lamp has the defect of having different luminous efficacies according to the lighting directions.